Secret telephony



June M, i946. l J. E* SMITH .2?401889 SECRET TELEPHONY n l Filed Sept. 18, 1942 Tl-ll ATTORNEY Patented june il,

sacaa'r :internem J. Ernest Smith, Jackson Heights, N. Y., assignor to Radio Corporation oi America, a corporation of Delaware Application September 18 1942, Serial No. 458,809

1l Claims. (Cl. 17g-1.5)

This invention relates to secret telephony and has for its object to provide in a telephone system a certain degree of privacy so that without the aid of special receiving apparatus, an unauthorized receiving station would ordinarily fail to recognize the intelligence transmitted.

It is an object of my invention to provide a telephone' system which may be operated over wire or radiochannels and in which secrecy i's maintained by virtue of a superposition of one train of intelligence signals over another by successive recordings at different velocities of the recording medium; and t provide facilitiesfor play-back and transmission of the record at a certain velocity dierent from either of the recording velocities.

It is another object of my invention to provide a secret telephone system in which diierent trains of modulations are superimposed upon a record. this record to be subsequently reproduced at a frequency different from either of the two recording frequencies for modulating a high frequency carrier wave.

As will be shown by way of illustration of a specific embodiment, my invention leads to possibilities for secret telephony by the use of very simple forms of apparatus and by the practice of methods which are substantially fool-proof and free from costly servicing of the equipment which would be utilized.

Various objects and advantages of my system other than those mentioned in the foregoing paragraphs will be made apparent in the more detailed description to follow. This description is accompanied by a drawing, in which:

Fig. 1 represents diagrammatically a telegraphone system including associated apparatus and circuits for carrying out the invention ;1 and Fig. 2 shows diagrammatically a preferred arrangement of receiving apparatus.

It is an important feature of my invention to provide facilities for making voice recordings at two diierent linear velocities of the record medium. These recordings are preferably superimposed upon a telegraphone wire, tape, or other recording medium, so that when reproduced at either of the recording velocities without suitable filtering, a, scrambling of the speech signals with noise signals results. When such superimposed records are reproduced at a third velocity of the recording madium, the output becomes still more unintelligible. t

As an example, let us suppose that two voice recordings Sl and S2 are made with ldelity bands from 50 cycles to 3 kc. each'. For intelligi- Y ble reproduction of 'these signals it is mandatory that the speed of reproduction be substantially equal to that of the original recording.

Suppose now that the recording velocity of the telegraphone tape is made 10 times as great for S2vas for Si, then in the reproduction of SI at its original velocity, the noise accompaniment of S2 will occupy an audible frequency band, say, from 5 cycles to 300 cycles. Also in the reproduction of S2 at its originabvelocity, the noise accompaniment of Si will occupy an audible fre quency band from 500 cycles to the upper limit of audibility, say 6 kc. y y

Assume now that the reproduction of the superimposed record is made at a third velocity, say. three times as great as the recording velocity of the slow speed. No recognizable signals whatsoever would then be obtained because of the wide departure of all voice signa1 components from their `normal range of frequencies. It is true that the signals might be recorded at an unauthorized station and played back at different trial velocities while applying certain filtering processes. However, the proper play-back velocity, fthe frequency band to be passed, and the frequency band to be suppressed would not be known, and the coordination of these variables for restoring intelligibility would be determined only by tedious experimentation. If the velocity of the telegraphonerwhen transmitting the superimposed records is made exactly three times as great as the slow velocity of recording, then Si will occupy a band from 150 cycles to the highest audible fre quency, say, 6 kc., and S2 will occupy a band from the lowest audible frequency up to 900 cycles. Cross-talk then exists throughout an overlapping band from 150 to 900 cycles. Nevertheless, after reception and recording of the combined signals at an authorized station, play-back of this record at the original recording velocity of Si causes a large quantum of th'e speech components of the S2 train to be moved into the infra-audible range, and the residue may be considerably suppressed by a high-pass filter whose low cut-oli point is set in the region of 300 `cycles or less. Filter means serviceable in this connection are well known in the art and need not be described in detail in this specification.

covei` a band from 500 cycles up to the upper limit of audibility. Hence, it may be preferable to record the wanted intelligence at the lower of the two original velocities.

Referring now to Fig, 1, I show therein a microphone or telephone transmitter which may be connected through contacts h and f of a switching arrangement 2| to the input circuit of an amplifier 2. This circuit may conventionally include a biasing battery 3, if desired. The output circuit from the amplifier 2 includes an operating potential source 4, the negative terminal of which is shown vconnected to a movable contact g in a switching device 2 I.

A telegraphone unit is shown generally at and may comprise a pair of drums 8 and 1 on which -a magnetic wire or tape 8 can be wound.

The tape 8 is arranged to pass between the pole pieces of a recording and pick-up magnet which possesses a winding 9. Pulleys I2 on the shafts of drums 6 and 1 may be belted together, as by means of a belt I3, so as to wind the element 8 off of drum 6 and onto drum 1, or vice versa.

The shaft of drum 1 carries another pulley II which is driven by a belt I8 and drive pulley I0. If `the magnetic tape 8 isiiat so that it must be wound in overlapping layers, then a friction clutch is provided between one or the other of the pulleys |2 and the winding drum which is coaxial therewith, If, however, the magnetic recording medium 8 is in the form of a wire, then a cylindrical helix wound on one drum and a similar helix unwound from the other drum will require a uniform speed of rotation of the two drums` which can be maintained without resort to friction clutches. y

Power for feeding the magnetic tape 8 is derived from a motor I4 which may be coupled ondary winding 24 is connected to any suitable transmitter, not shown.

' Switch 20 comprises movable contact springs a and b which are sandwiched between stationary contact springs c, d, and e. The switch 2| is similarly composed of movable contact springs f and g sandwiched between stationary contact springs h, i, and k. The connections of these springs and the functions which they perform will be referred to hereinafter in the description of operation,

For recording a train of speech waves while the magnetic tape 8 is driven at low speed, the microphone I is connected through contacts h and f and through battery 3 to the input circuit of the amplifier 2, both ends of this circuit being grounded. The output circuit from the amplifier 2 may be traced through the source 4` through contacts g and i, and thence through contacts d and a to the coil 9 of the telegraphone, from which the return circuit is traced through contacts b and e to ground.

For recording a second train of speech waves while the magnetic tape 8 is driven at high speed,

. the same connections are made as described in the through a shaft |5 and coupler member I6 to a gear mechanism (not shown) within a gear box I1. The power is delivered from the gear box by the pulley I0 which drives the belt I8.

In order to carry outmy invention in a practical xnanner, it is desirable to run the magnetic tape 8 at three forward speeds rand one reverse speed. The use of a gear shift mechanism equivalent to that, in automobile is thus suggested. Such a gear shift mechanism may be provided in I the unit I1 and includes, for example, a gear shift lever I9 having a conventional choice of positions as indicated by the letter H, Where the upper left arm is designated R. for reverse; the lower left arm is designated L for low gear; the upper right arm is designated I for intermediate gear: and the lower right arm is designated H for high gear.

The low and high gears are used for recording, and the intermediate gear is preferably used for play-back and transmission. The reverse gear is used when back-winding between two recordings and when erasing the record. y

Two multi-contact switches 20 and 2| are provided, Switch 20 is mechanically actuable by the gear shift lever I9 when it is moved into the reverse position. Switch 2| (shown in its actuated position) is controlled by moving the gear shift lever I9 into intermediate gear. When the gear foregoing paragraph. Between the two record-` ings, however, it is necessary to wind the magnetic tape backward without erasing its first record. The gear shift lever, therefore, is placed in reverse and switch 25 is open, this switch being normally closed at all other times.

Assuming now that the two superimposed speech Vrecords are to be played back and transmitted over a communications channel connectedv to the secondary of transformer 23, the gear shift lever I9 will now be moved to the intermediate position, whereat contacts f and g of switch 2| occupy the positions shown in Fig. 1. Under these conditions, the microphone is disconnected and the electrical waves generated in the circuit of coil 9 by the magnetic influence of the tape 8 will be fed into the input circuit of the amplifier 2, as may be seen by tracing this input circuit as follows: Starting at the amplier, this circuit includes source 3, contacts f and i to contact d, which at this time is engaged with contact a and is in circuit with the winding 9. The return circuit goes through switch 25. and contacts b and ey to ground. The output vcircuit, from amplifier 2 may also be traced shift lever I8 is thrown into low gear or high through source 4 and contacts g and k tothe primary winding 22 and thence to ground. The superimposed recordings which have been made on the magnetic tape 8 will, therefore, be capable of transmission by 'suitable connection of the transformer secondary 24 to a transmitter.

When the transcriptions are to be erased from the magnetic tape 8 then the gear shift lever I9 is moved into reverse position and switch 25 remains closed. Under these conditions a strong magnetizing current from source 4 is caused -to polarize the lines of force in the magnetic tape 8 oppositely ,from the polarization of the sound wave recordings. The demagnetizing circuit may be traced as follows: From source 4 it leads through conductor 26 and a suitable resistor 21 to contacts c anda (now in engagement) and thence through winding 9, switch 25, and contacts b, e, 7', and g. to the negative terminal of source 4. I y

'It will thus be seen from the foregoing description that with the aid of the gear shift lever I9 and the cooperating switches 20, 2|, andv 25, the'various steps of recording, play-back, rewinding, and erasure of the transcriptions may be conveniently performed at will.

accise In order to restore the substantially unintelligible transmissions of speech waves to intelligibility, a receiving station may be provided as schematically represented in Fig. 2.

The equipment at the receiving station preferably includes a radio receiver` 28 fed with energy from any suitable antenna 29. The output from the receiver is utilized in a sound recorder 30. The recording speed is preferably at the same number of feet per minute of tape feed as is obtained at the transmitter during the playback process.

By the use of a record player 3i' driven at a l speed corresponding to the original speed of recording of one of the superimposed speech records. it is possible to perform the essential step of restoring intelligibiiity. Either of the two transcriptions may contain the wanted intelligence. The record player 3| may, therefore, be provided with two playing'speeds, if desired, so as to restore either of the original transcriptions to intelligibility.

The differentspeeds of the transcriptions may be such that the voice frequency band of one record does not overlap that of the other. On the other hand, it may be desirable to introduce a certain degree of cross-talk for the sake of greater secrecy of transmission.- In this case.

the overlapping voice frequency bands of the two transcriptions may be such that cross-talk will notbe entirely eliminated by the use of a, filter such as shown by the unit 3'2. Preferably, however, the filter 32 is arranged for shifting its cutoff point so as to obtain an optimum degree of selectivity of thetranscription which is wanted. and so as to suppress as far as possible the reproduction of the unwanted record.

The output from the lter amplifier 32 may` be fed to a conventional loud speaker e3, or in place thereof to a set ci head phones, not

and in the play-back of the records. In all cases however, the original frequencies are restored as wanted by the use of selective speeds in the record player 3l and by proper choice of the cut-ofi point in the filter 32. The satisfactory determination of speeds and cut-off points may be known only to the operator of the authorized receiving station, whereas at unauthorized stations, it would be a matter of tedious trials of different speeds and different cut-off frequencies before the intelligibility of the reception could be obtained. Bearing in mind that two speech waves are superimposed one upon another, and in the transmission neither of these Waves appears at normal frequencies, it will be obvious that the difclty of unscrambling the 'intelligence at an unauthorized station would be very great.

I claim:

l. In a secret telephone system, a sound recording medium, means for moving said medium past a recording point at .r feet per minute, and subsequently at y feet per minute, means for recording on said medium during its travel at feet per speech waves which is superimposed upon the i at z feet per minute, velocity z being intermediate between the two recording velocities z and y and the recording velocities having a dierence such that appreciable overlap of speech frequency bands is obtained at the transmitting velocity a.

2. In a secret telephone system, a sound recording medium, means for moving said medium past a recording point at z feet per minute, means for recording on said medium while it travels at velocity z a signal train which is composed of two superimposed but frequency-shifted trains of speech Waves, the frequency shifts corresponding to velocity diierences (rr-e) and (z--y), where velocities a', y and z are the same as applied to the movement of the recording medium in the apparatus defined by claim 1, a reproducing device subject to control by moving said sound recording medium past a pick-up point, means for causing said reproducing device to emit speech waves having-"the natural ,frequencies of one of said trains of speech waives, and means for bandpass filtering the emission of said speech waves in such manner that the cross-tallr'eflects of the superimposed train of speech waves is substantially eliminated. l

3. In a system of telephony the method which comprises transcribing successively two trains of speech waves, causing thevelocity of the record medium to be substantially different for the two transcriptions, causing 'one transcription to be superimposed upon the other with' their respective frequency bands overlapping, playing baci: the combined transcriptions at a third velocity of said record mediumintermediate between the two recording velocities, transmitting the play-back over a communications channel, remotely receiving and recording the transmission, reproducing -the sound waves of one of said two trains at their original frequencies, and filtering out a substantial band of unwanted noise components attributable to the other of the superimposed wave trains.

4. A secret telephone system comprising a telegraphone having a linear magnetic recording medium and magnetic recording and play-back.

on said medium, said amplifier having input and output circuits alternatively connectible to said recording and play-back means, manually operable means :for selecting the desired velocity at which to drive the recording medium, and switching means operatively associated with said manually operable means for setting up suitable cir- .cuit connections through said ampliiier either for dium which diifers substantially from both of two recording velocities thereof.

5. .The method of secret communication which comprises making a phonographic record of the sound waves constituting a piece of intelligence to be communicated, super-imposing a second rec ord of sound waves on the first while passing the record medium through the recording position at an altered velocity, the frequency band of the second record having a partial overlap relation to that of the first record, translating the combined record into electrical waves the frequency band of which lies intermediate the frequency bands of the first and second records, transmitting, remotely receiving and recording said electrical waves, reproducing sound waves from the recorded transmission but at frequencies correspondingl to those of the rst record, and filterblocking unwanted frequency components of the recorded transmission which lie substantially outside the frequency band of said first record.

6. Apparatus for secret telephone communica'- tion comprising sound recording means operable at two different linear recording velocities to superimpose one train of speech waves upon another, the difference between said velocities being such that a substantial overlap occurs between the frequency bands of the two trains with any play-back velocity. record play-back means operable at a.- third linear velocity intermediatev the first two velocities, telephone transmitting equipment arranged for control by said play-back means,v remotely situated telephone receiving equipment including recording means responsive to signals delivered by said transmitting equipment, sound reproducing means for translating the products of the last mentioned recording means into electrical Waves having superimposed components such that one of said components corresponds with sound frequencies of the intelligence to be communicated, and the other of said components is a frequency-shifted derivative from other sound waves recorded for transmission, and means for filter-blocking a band of audio frequencies of said other component which is substantially non-overlapping with respect to the band of said one component.

7. Apparatus according to claim 6 and including a motor and manually selective gear-shift mechanism for driving the first said recording means at different speeds and in opposite directions.

8. Apparatus according to claim 6 andincluding an ampliiier', and selective means for at times causing said amplifier to transfer amplifled sound frequency waves to the first said recording means, said selective means being operable at other times for causing said amplifier to transfer amplified signals from said play-back means to'said transmitting equipment.

9. A phonograph recording and play-back system comprising an audio-frequency responsive device adapted and arranged to record speech waves, a record medium, means for moving said record medium at dierently selected velocities through the recording position of said device, said velocities being so related that upon play-back of said record medium there is appreciable crosstalk in the absence of filtering, means for causing the superposition of onespeech wave train upon another on said record medium, the two trains being differentiated by different velocities of the record medium, and play-back means operable under control o! said record medium while moving the same at a velocity different from either of the recording velocities.

1-0. A system in accordance with claim 9 and including motor driven selective gears for determining the velocity oi motion of said record medium, land a manually operable gear-shift device for selecting a desired speed and direction of travel of said medium.

11. A system in accordance with claim 9 and including amplifier means adapted and arranged to amplify speech waves, means for connecting said responsive device to the output side of said amplifier during the recording vof said wave trains, and means for connecting the input side of said ampliiler to said play-back means during the reproduction of the combined wave trains of said record medium.

J. ERNEST SMITH. 

